Numerical and experimental investigation of the stability of radio-frequency (RF) discharges at atmospheric pressure

Abstract

The stability and uniformity of a radio-frequency (RF) discharge is limited by a critical power density. Beyond this critical power density, instability occurs in the form of physical changes in the plasma (such as contraction due to arcing). The RF discharge used in this study is the non-equilibrium Atmospheric Pressure Plasma Jet (APPJ ® ) developed by Apjet, Inc. This discharge is known to operate uniformly in helium gas. However, for some proposed applications such as surface modification, there is a need to operate with reactive gases such as O 2 . Our experimental studies show that addition of molecular gas to a discharge operating in helium increases its power density (W cm −2 ), until it reaches the critical unstable arcing limit. Moreover, an increase in the frequency of operation (from 13 to 27 MHz) allows the plasma to sustain higher molecular gas concentrations and power densities before instability occurs. Further, it is observed that this critical power density is dependent on the type of molecular gas added. These results provide a motivation for the development of a mathematical model that can provide insight into the causes of instability and potential methods of suppression. The two commonly studied modes of instability are (1) thermal instability (TI) and (2) α-γ -arc mode transition. For the APPJ ® discharge conditions, the development time scales of TI are much longer (∼1 ms) as compared with discharge oscillation period (∼100 ns). Hence, if the instability was indeed thermal, discharge frequency increase would have no consequence, contrary to experimental findings. A 1D fluid model based on the local field approximation is developed to study instability in APPJ ® discharge. The analysis of modeling results confirmed our hypothesis that the instability development actually takes place via breakdown of sheath i.e. α-γ -arc mode transition and not by TI.

Stochastic and Nonlinear Dynamics in Low-Temperature Plasmas

“…Several kinds of instabilities have been identified, the thermal instability and the α to β state transition, being the two most encountered. The thermal instability occurs in a sequence of processes [20] leading to a (positive) fluctuation of the electron density δn e which increases the electric current density δ j e . It results in an increment of power dissipation δ( j e E ).…”

“…It must be mentioned that, in fact, the described amplification process can be initiated with a fluctuation of any of the involved physical properties. The discharge is unstable if the rate of heating of the working gas, ν h , is higher than the cooling rate, ν c , divided by a factor, ν * , known as the logarithmic sensitivity, given by [20]:…”

Investigation of the Atmospheric Helium Dielectric Barrier Discharge Driven by a Realistic Distorted-Sinusoidal Voltage Power Source

“…, He þ 2 , He) and 27 reaction channels, with one more species (He ÃÃ ex ) than the simple set. In general, the complicated plasma chemistry includes more reaction channels related to heavy particles, referring to Table 1, such as ion-e recombination [12], ionelectron dissociative recombination [14], ion-electron recombination [15,16], HornbeckMolnar associative ionization [17], metastable-metastable associative ionization [18], metastable-induced dissociative association [22], metastable-induced association [23], dimer-induced dissociative ionization [24], dimer-induced ionization [25] and helium-atom induced dissociation [26].…”

“…On one hand, in the case of RF AP discharge, one-dimensional fluid modeling was often used to elucidate the plasma physics by varying parameters such as the gap distance, the amplitude of the sinusoidal voltage, the frequency of the sinusoidal voltage, the mixture of impurities and the secondary electron emission coefficient [10][11][12][13][14]. In these studies, very simplified helium plasma chemistry was used and generally was in reasonable agreement with experimentally measured discharged currents.…”

Spin–lattice coupling in iron-pnictide superconductors: a model for possible continuous phase transition

“…It is seen that the electric field and the potential changes periodically versus position. The root mean square of the electric field can be used to investigate the sheath region [17,18].…”